For example, in a patent document 1, in an inline four cylinder internal combustion engine, there has been disclosed an exhaust device having a configuration in which exhaust ports for a #2 cylinder and a #3 cylinder whose ignition orders are not sequential merge inside a cylinder head and exhaust ports for a #1 cylinder and a #4 cylinder are directly opened on the side surface of the cylinder head. That is, the exhaust ports for the #2 cylinder and the #3 cylinder are configured as a single collective exhaust port, and the exhaust port for the #1 cylinder and the exhaust port for the #4 cylinder are configured as an individual exhaust port independently provided for each of the cylinders. In addition, the collective exhaust port for the #2 and #3 cylinders is connected to a catalytic converter through a single collective exhaust pipe, and individual exhaust ports for the #1 cylinder and the #4 cylinder are connected to the catalytic converter through an independent individual exhaust pipe in each of the cylinders. In the patent document 1, the leading end parts of these collective exhaust pipe and individual exhaust pipes are connected to the end part of the catalytic converter so as to be basically parallel to the central axis of the catalytic converter.
In this way, in the configuration in which the exhaust ports for some cylinders merge inside the cylinder head, at the time of cold start, an exhaust gas at a high temperature which is introduced to the catalytic converter through the collective exhaust pipe can be obtained, and consequently, there is an advantage in early activation of a catalyst after starting the internal combustion engine.
However, on the other hand, the flow velocity of the exhaust gas introduced to the catalytic converter through the collective exhaust pipe and the flow velocity of an exhaust gas introduced to the catalytic converter through the individual exhaust pipe are different. That is, the passage cross sectional area of the collective exhaust pipe, in which the exhaust ports for the #2 and #3 cylinders merge, is set larger than that of the individual exhaust pipe for each of the cylinders, and the flow velocity in the collective exhaust pipe is relatively slow. With this, the exhaust gas introduced to the end part of the catalytic converter spreads out to a certain extent and reaches the end surface of a catalyst carrier. On the other hand, the flow velocity of the exhaust gas introduced from each of the individual exhaust pipes for the #1 cylinder and the #4 cylinder is high and the rectilinearity of this gas is high, and consequently, the exhaust gas locally collides with a part of the end surface of the catalyst carrier.
In addition, as compared with the temperature of the exhaust gas which flows into the catalytic converter from the collective exhaust pipe, the temperature of the exhaust gas which flows into the catalytic converter from each of the individual exhaust pipes generally becomes low.
Therefore, for example, flow velocity distribution and temperature distribution in the catalyst carrier configured as a monolithic catalyst carrier easily become non-uniform, and the early deterioration of a catalyst and cracks in the catalyst carrier caused by temperature difference are concerned.